SummaryWe have recently studieda case series of 46 unrelated patients with inherited thrombocytopenias and identified 18 cases that did not fit any known platelet disorder. In two unrelated families, a mild thrombocytopenia with normal platelet size was transmitted in an autosomal dominant fashion. Bleeding time was prolonged in 5 investigated patients. In all of them, flow cytometry and SDS-PAGE of platelet glycoproteins (GP) showed a reduced content of GPIa, a subunit of the GPIa-IIa complex (also known as integrin α2β1) that is a major collagen receptor on platelets.All other membrane GPs were within the normal range. GPIa deficiency was associated with severely reduced in vitro platelet adhesion to molecules known to interact selectively with GPIa. In vitro platelet aggregation was normal in all subjects, except for a suboptimal platelet response to fibrillar collagen in two patients.A mild defect of α-granules was observed in all affected subjects. No mutation was identified in the genes encoding for GPIa or GPIIa. Since no other similar cases have been reported in the literature, we suggest that an autosomal dominant thrombocytopenia associated with GPIa deficiency and α-granule defect represents a new form of inherited thrombocytopenia.